Honokiol is one of two dominant biphenolic compounds isolated from Magnolia spp. bark, and is the most widely researched active constituent of the bark. In this literature review we discuss the accumulating body of preclinical research which shows honokiol to have wide-ranging biological and clinically relevant effects, without appreciable toxicity. In vivo studies suggest that honokiol’s greatest value is in its multiple anticancer actions. In vitro research suggests honokiol has potential to enhance current anticancer regimens by inhibiting angiogenesis, promoting apoptosis, providing direct cytotoxic activity, down-regulating cancer cell signaling pathways, regulating genetic expression, enhancing the effects of specific chemotherapeutic agents, radio-sensitizing cancer cells to radiation therapy, and inhibiting multidrug resistance. Honokiol also shows potential in preventative health by reducing inflammation and oxidative stress, providing neurological protection, and regulating glucose; in mental illness by its effects against anxiety and depression; and in helping regulate stress response signaling. Its antimicrobial effects demonstrate potential for partnering with antiviral/antibiotic therapy, and treating secondary infections. Honokiol may occupy a distinct therapeutic niche because of its unique characteristics: the ability to cross the blood brain barrier (BBB) and blood cerebrospinal fluid barrier (BCSFB), high systemic bioavailability, and its actions on a multiplicity of signaling pathways and genomic activity. There is a need for research on honokiol to progress to human studies and on into clinical use. Currently, honokiol is used by a growing number of practitioners in integrative treatment protocols for cancer and other conditions.

IntroductionMagnolia bark has been extensively used in Traditional Chinese Medicine (TCM) for treating symptoms due to “stagnation of qi,” as well as stress-related symptoms, including digestive disorders arising from anxiety and emotional imbalances. It is a common ingredient in a number of TCM formulas, with detailed indications in TCM literature beyond the scope of this review. It is also widely used in Japan as an anxiolytic in the form of tea. Magnolol, the other main active phenolic compound in magnolia bark, shares some biological properties with honokiol and has been independently researched.

Anticancer ActionsThe preclinical research on honokiol’s broad-ranging capabilities shows its potential as a therapeutic compound for numerous solid and hematological cancers, including its effectiveness in combating multi-drug resistance (MDR) and its synergy with other anticancer therapies. Research thus far shows no toxicity or serious adverse effects in animal models. There is currently no clinical data on the use of honokiol in humans.

Crosses the blood-brain barrierIn vitro research suggests that honokiol crosses the BBB and may reach therapeutic concentrations in the brain via passage through the tight junctions formed by cerebral endothelial cells (CECs). In the same experimental study, honokiol reached brain tissues following intravenous injection in rodents (25 mg/kg). Ultimately, this study demonstrated that honokiol was able to induce apoptotic insults to neuroblastoma cells through a Bax-mitochondrion-cytochrome c-caspase protease pathway, at concentrations that traversed the BBB. Though the exact mechanism of honokiol’s ability to cross the BBB has not been fully clarified, honokiol has been shown to down-regulate P-glycoprotein, whose expression is critical to efflux pump regulation. Overexpression of P-glycoprotein is one of the primary mechanism of MDR. In rodent models of brain tumors, honokiol showed growth inhibition when given intravenously.

Promotes apoptosisOne of the hallmarks of cancer is the failure of apoptosis (programmed cell death), which enables abnormal cells to replicate uncontrollably. Many chemotherapy drugs and natural anticancer compounds act in part by inducing apoptosis or restoring the cancer cells’ ability to trigger apoptosis. Research shows honokiol induces apoptosis via multiple primary pathways involved, suggesting a therapeutic advantage over compounds that target single pathways.

A number of studies have been published to clarify our understanding of the biochemical pathways influenced by honokiol. A comprehensive 2011 article by Xu et al in Drug Discoveries and Therapeutics reviewed the multiple apoptotic pathways by which honokiol exerted effects on apoptosis, and also reviewed the multiple cells types that have shown inhibition in in vitro and in vivo studies.4 Cell types include breast, prostate, colon, liver, squamous cell lung cancer, and chronic lymphocytic leukemia (CLL). A study published in PLoS One in 2011 found that honokiol arrested the cell cycle and induced apoptosis in pancreatic cancer cell lines. In non-small cell lung cancer, honokiol suppressed cancer cell growth and induced apoptosis through influence on multiple cell-signaling pathways.6 In the treatment of leukemia, honokiol induced cell cycle arrest and apoptosis through the inhibition of specific cancer cell survival signals. A 2012 study showed honokiol’s ability to stop the proliferation and spread of malignant melanoma. In this study, honokiol induced cancer cell death and blocked proliferation by regulating cell cycle arrest through multiple signaling pathways. Another in vitro study showed that honokiol effectively induced cell cycle arrest irrespective of the androgen sensitivity status of prostate cancer cells.

Inhibits angiogenesis Angiogenesis, another multi-pathway signaling process, becomes up-regulated in cancer cell growth and metastasis. The tumor sends out chemical signals that stimulate a transformation in endothelial cells that line nearby blood vessels, encouraging them to branch and grow toward the tumor, providing blood and nutrient supply for rapid growth. As with apoptosis, there are multiple pathways that can be targeted with specific therapies to interrupt or inhibit tumor angiogenesis. Research has elucidated that honokiol accomplishes angiogenesis inhibition through modulation of NF-kB pathway.

Honokiol has also been shown to inhibit spread of cancer cells through the lymph system by inhibiting one of the primary pathways involved in growth stimulation related to vascular endothelial growth factor (VEGF).

A 2012 in vivo study in PLoS One showed that honokiol, by inhibiting angiogenic pathways such as STAT-3, dampened peritoneal dissemination of gastric cancer in mice (5mg/kg delivered intraperitoneally).

Crosses the blood-brain barrierIn vitro research suggests that honokiol crosses the BBB and may reach therapeutic concentrations in the brain via passage through the tight junctions formed by cerebral endothelial cells (CECs). In the same experimental study, honokiol reached brain tissues following intravenous injection in rodents (25 mg/kg). Ultimately, this study demonstrated that honokiol was able to induce apoptotic insults to neuroblastoma cells through a Bax-mitochondrion-cytochrome c-caspase protease pathway, at concentrations that traversed the BBB. Though the exact mechanism of honokiol’s ability to cross the BBB has not been fully clarified, honokiol has been shown to down-regulate P-glycoprotein, whose expression is critical to efflux pump regulation. Overexpression of P-glycoprotein is one of the primary mechanism of MDR.2 In rodent models of brain tumors, honokiol showed growth inhibition when given intravenously.

Additional ApplicationsAnti-inflammatory effectsHonokiol demonstrates systemic anti-inflammatory effects via multiple mechanisms. Primarily, these effects relate to some of honokiol’s anticancer and anti-MDR activities, including inhibition of TNF-α stimulated NF-kB activation, and NF-kB–regulated gene expression. By inhibiting the NF-kB pathway, honokiol inhibits nitric oxide (NO) generation. These effects were observed in vivo systemically as well as topically. In vitro, honokiol reversed CD40- and LMP-mediated NF-kB and AP-1 activation and suppressed TNF-α and IL-6 production in mouse B-cell lines. Honokiol is shown to reduce NF-kB target genes such as VEGF, ICAM-1, and COX-2. Honokiol appears to inhibit NF-kB activation through a number of stimuli.

In a murine model of arthritis, honokiol demonstrated significant anti-inflammatory effects. Treatment with honokiol decreased clinical scores of collagen-induced arthritis in both normal and transgenic mice. Furthermore, antibody production, particularly IgG3, was diminished together with IL-12, IL-6, interferon gamma, and, notably, IL-17. These finding suggest indications for the treatment of IL-17–mediated inflammatory disorders including rheumatoid arthritis, psoriasis, and inflammatory bowel diseases. Other studies show that honokiol also works against inflammation by inhibiting PI3k/Akt signaling pathways, as well as inhibition of downstream pathway of MEK in NF-kB signaling.

Antioxidant and selective pro-oxidant actionsThe free radical–scavenging effects of honokiol against reactive oxygen species are documented in multiple studies. In vivo research in a cardiac lipid peroxidation model demonstrated honokiol’s free radical–scavenging abilities, helping to protect rat heart mitochondria. Results of oxygen consumption and malondialdehyde production showed that honokiol inhibition of reactive oxygen was 1,000 times that of α-tocopherol (vitamin E). The antioxidant abilities are believed to be attributed to the allyl groups on honokiol. The antioxidant actions of honokiol against a number of conditions, including cardiac ischemic injury and hepatic peroxidative injury, are outlined in the 2009 review “Honokiol, a multifunctional antiangiogenic and antitumor agent,” by Fried and Arbiser in the Antioxidants and Redox Signaling.

Honokiol also functions as a selective pro-oxidant, generating reactive oxygen against cancer cells with wild-type p53 status. It is likely a result of and triggered by p53 mutation, and HIF/mTOR-1 activation of the mitochondrial permeability transition pore.

Anxiolytic effectsMagnolia bark plays a central role traditional in Chinese medical formulas as an anxiolytic. Honokiol extracted from the bark has been studied independently to investigate its mechanisms of action on the central nervous system and has been shown to interact with neurotransmitter gamma-aminobutyric acid (GABA) receptors. In vivo, honokiol showed similar anxiolytic effects to diazepam without a change in motor activity or muscle tone observed with diazepam. Honokiol showed no evidence of withdrawal symptoms, whereas diazepam withdrawal was characterized by hyperactivity. Diazepam also disrupted memory and learning, side effects not seen with honokiol administration. It is also shown to increase hippocampal acetylcholine release in vivo.

Neuroprotective effectsNeuroinflammation occurs via the activation of microglia in response to inflammatory stimuli with subsequent release of proinflammatory cytokines and prostaglandins, including TNF alpha, IL-6, and COX-2. An in vitro study in 2012 showed that honokiol reduces inflammation in brain tissue by down-regulating transcription factors that control the activation of overactive microglia, thus inhibiting the release of these inflammatory compounds.

Amyloid beta peptide (A beta)–induced toxicity is a well-established pathway of neuronal cell death, which might play a role in Alzheimer’s disease. A 2010 in vitro study found that honokiol significantly decreased A beta–induced cell death, possibly mediated through reduced ROS production, as well as suppression of intracellular calcium elevation and inhibition of caspase-3 activity.

Using a rodent model, honokiol was also shown to protect against damage from cerebral ischemia by reducing inflammation and oxidation in the brain. General neuroprotective activity against cerebellar granule cell damage has also been demonstrated.

ConclusionsA large body of preclinical research exists on honokiol. In vitro studies have investigated its multiple mechanisms of action, while in vivo studies have shown its potential benefit across a broad range of illnesses, including difficult-to-treat conditions such as MDR cancer. Honokiol’s in vivo toxicity record thus far shows it to be safe. It is also shown to cross the BBB.

The preclinical research on honokiol is compelling, and its potential benefits combined with lack of toxicity are promising. Honokiol is currently being administered by a growing number of clinicians as an adjuvant therapy to address numerous pro-inflammatory and neurologic conditions, as well as multiple types of cancer. There is an urgent need to progress to clinical trials in order to validate and further clarify honokiol’s diverse range of applications.